Mikrosensorer i nätverk
The purpose of this thesis is to investigate the possibility to make small, cheap and ultra low power sensor platforms, connected in wireless networks for monitoring of home and industrial environments. The concept is based on ideas from our supervisor, Jerry Lindblom. The developed system consists of a generic platform for different sensors, a receiver and software. A GPRS module from RO Rollytech AB and a database from Gema Industri AB have been modified and used to demonstrate an example of a complete system. To the platform, it is possible to connect any resistive sensor and any sensor that can act as a switch. It is also possible to use sensors compatible with the I2C interface. Parameters the can be monitored are, for example, moisture, temperature, vibrations, smoke. The range of possible products is probably larger than can be imagined. To make the life time of the module acceptable, the power consumption must be extremely low. This is possible to realize by letting the module sleep until it is waked by an external event. The sensor module wakes up and sends an alarm when, for example, the temperature goes below a certain level, or when fluid is detected under a freezer. The only message that is regularly sent is the ?I?m alive? message which is transmitted twice a day to confirm a working module. When the alarm or the ?I?m alive? message is transmitted the analogue value of the sensor is embedded in it. The sensor modules communicate on the 868 MHz frequency band which is license free. This band is used for surveillance systems and measurement data collection. The modules use FSK modulation and a protocol developed by us. It has been shown that the protocol supports multiple senders. A complete system was demonstrated at the Technical Exhibition in Stockholm, at this year prize ceremony for the Embedded Student Award. The demo showed how the information from the sensor network can be reached by a central server which has a database and tools for data processing. The user can, through his website, create graphs to show trends and set up conditions for alarms. The user can also get fast information about alarm status, by e-mail or SMS. Crucial parts of the thesis were to consider the manufacturing cost, size of the platform and the power consumption. The manufacturing cost is today approximately 45 SEK including battery and a basic sensor which is considerably cheaper than is available on the market today. The goal to develop a small sensor platform has been achieved. The diameter of the circuit board is only 21 mm. The power consumption has been minimized through a reactive method of programming which has been implemented in C. The total power consumption is approximately 85 uAh/day. For obvious reasons an endurance test was not possible but when 85 uAh/day is the actual power consumption, a qualified guess of the life time is 4 years with a 150 mA battery.